Several kinds of electron beam exposure systems are known in the art. Most of these systems are provided to transfer very precise patterns onto an exposed surface of a substrate. Since lithography features are pushed to become smaller and smaller following Moore's law, the high resolution of electron beams could be used to continue the drive to even smaller features. Electron beam lithography, metrology and inspection systems have historically suffered from low throughput and limited focal length and resolution. A conventional electron beam exposure apparatus has a throughput of about 1/100 wafer/hr. However, for lithography purposes a commercially acceptable throughput of at least a few wafer/hr is necessary. For lithography, metrology and inspection systems resolution of nanometer-scale features requires a beam spot size significantly smaller than the features being resolved. To achieve such small spot sizes, smaller focal lengths are often desirable.
One possible solution to these limitations involves an approach using many beams writing simultaneously. For example, U.S. Pat. No. 6,897,458, U.S. Pat. No. 7,091,504 and U.S. Pat. No. 7,804,414 disclose multi-beam projection columns for mask-less electron-beam lithography and inspection. These multi-beam projection columns are composed of carefully aligned arrays of apertures and entirely electrostatic focusing and deflection elements. Multiple electron beams are formed by illuminating an array of apertures with electrons from a high current, high brightness electron gun which typically has a large area cathode. Alternatively, multiple discrete electron sources can be used but these tend to be difficult to control and align.
For single-column multi-beam systems, including projection optics and blanked apertures arrays, throughput, focal length and resolution may be limited by space-charge effects. The smallest achievable beam diameter for useful current (about 1 nA) in a conventional multi-beam system may be on the order of tens of nanometers, limited mainly by chromatic aberration in the electrostatic projection lens arrays. The chromatic aberration coefficient of the lens scales with the focal length. However, the focal length of such lenses can not be reduced beyond a certain practical limit or the electric field strength between lens electrodes becomes impractical to maintain without flashover.
It is within this context that embodiments of the present invention arise.